There are very good reference works on surface plates and mounting them. If you 'fully' support the bottom by potting it in the floor compound, it will not be as per the calibrated accuracy. If you only care about thousands, and don't mind your plate being a few thousands out, then got for it! A good surface plate (not shop grade) will be locally accurate in tens of millionths of an inch, and over the entire surface in tenths of a thousand of an inch.
I am trying to answer the question -correctly-, there are a lot of ways to 'make do'. A pivot is on one side of the plate to ensure that all four contact points have exactly the same pressure on them. This way the plate will experience minimum flex if they are on the 'Bessel' points. I prefer to build once, the best I can, so I don't have to do it over. Oh and yes, an improperly supported plate will flex and you can see it on a tenths indicator.